The present invention relates to analytical instruments for detecting and identifying trace levels of selected vapors, and in particular to portable instruments.
This invention relates to detection and identification of various vapors in gaseous media, such as air. The invention relates to hazardous gas detection, but has particular application to the detection of explosives and other illegal substances in the baggage and cargoes of boats, aircraft, or other vehicles.
The detection of explosives in baggage and other cargo presents a formidable challenge to present detection technologies. The room temperature equilibrium vapor pressures of such explosives as cyclonite (RDX) and pentaerythritol tetranitrate (PETN) are 1.1.times.10.sup.-9 torr and 3.8.times.10.sup.-10 torr, which translates to 1.4 ppt (parts per trillion) and 0.5 ppt, respectively, at atmospheric pressure. Another explosive denoted as HMX, has a room temperature equilibrium vapor concentration of about 0.1 ppt at atmospheric pressure. Even the more common explosives, such as nitroglycerin (NG), dinitrotoluene (DNT) and trinitrotoluene (TNT), with the relatively high equilibrium vapor concentrations of 300 ppb (parts per billion), 140 ppb and 6 ppb, respectively, may be expected to give off much lower concentrations, probably in the sub-ppt range, in the space surrounding a well-packaged explosive, especially if the package had been introduced into that space less than an hour earlier. Not only will the vapor get highly diluted as it diffuses into the surrounding space, but also most of that vapor will get lost by adsorption onto the surrounding materials. These dilution and adsorption effects may be expected to bring down the actual concentrations of the low-vapor-pressure explosives, such as RDX, PETN or HMX, down to 1 ppq (part per quadrillion) or less.
It is known to preconcentrate analytes in air samples by the use of sorbents, such as charcoal, Tenax.TM., or silica gel, but such techniques have the disadvantage of requiring desorption and sorbent-reconditioning steps, and they also risk possible introduction of interfering contaminants from the sorbents, analyte breakthrough, and the like.
A preconcentrator-sampler has been developed for use with a highly sensitive ion mobility spectrometer, and is based on adsorption of explosive vapors on quartz, followed by their thermal desorption at about 140.degree. C. But the possibility of thermal decomposition of some of the explosives molecules at or near 140.degree. C. is a cause for concern and renders the device unsuitable for the detection of those compounds that tend to decompose at temperatures above 100.degree. C.
Neutron activation has been used to detect the atomic composition of a concealed explosive, but this may be masked by harmless substances of similar atomic composition.
The aforementioned copending U.S. application Ser. No. 892,990 discloses the use of an absorption preconcentrating air sampler to increase the sensitivity of an analytic instrument. In such a sampler, a substantial portion of the analyte contained in a large volume of air becomes absorbed in a small volume of liquid extractant that can be injected directly into an analytic instrument, such as a liquid chromatograph. As compared with other methods, the direct absorption of an analyte from an arbitrarily large volume of air into a small volume of liquid extractant offers the advantages of low-temperature operation, simplicity, speed and flexibility. While that device has proved effective for detecting hazardous analytes, such as highly carcinogenic primary aromatic amines, at ppb concentrations, it is not suitable for detecting vapors at sub-ppt levels.